Rounding Off Numbers...

As you may know, I have a shaving cream project I have been working on for the last three months.  In reviewing everything, I have found that some of my problems with this formula have come from rounding off some of the numbers from within the formula.  My original formula did not have enough of the bases (hydroxides, TEA) to full saponify the oil phase, I also added a them as well as a preservative, EDTA and I added the required amount to the formula to get the results I wanted, but it brought the formula up to around 108%.  Some of the numbers added to the original formula were the required numbers for a 100% formula and needed to be higher to work with 108% total.  My solution was to try to get everything up to 110% which I thought would make it easier to reduce it to 100% number-wise.  Part of the problem was that in adding extra ingredients, I used the percentage required for a 100% formula but I actually needed more to provide enough of each of those ingredients to work with the 110% amounts.  The problem lies with the resultant percentages with all those extra decimal points!  One example would be an amount that came up to 41.398%  which I rounded off to 41.4%...not a problem but when I got to the end of rounding off all these numbers, I had to add .75% to the water phase.  It many not be a problem but keeping the oil phase and water amounts within a certain range does affect the end result in this product which you may have read created one version of my product that went back and forth between solid and liquid!

So how do you guys do it?  It has turned out to be quite a balancing act and I'm really close but I want to understand better how this should be approached.  Does one of Perry's videos address this issue or is there some basic cosmetic chemistry book that may cover this?  Your input will be greatly appreciated!

Tagged:

• I just use Excel to keep things on track.
Design of anti-aging creams, gels, and serums; shampoos; and therapeutic cosmetics. In-house label and box design capability.
• I use an amount to percentage online calculator which works well for me.

http://www.ingredientstodiefor.com/item/Amount_to_Percentage_Calculator/873?category=115#

My question is just how far should I go?  There is a balancing act going on and I just want to make sure I'm not going too far to make the numbers "work" in percentages!  When I'm trying to get it to 9.0% and it actually would be better if it were 9.2% ...along those lines...  Once I have this basic cosmetic chemistry approach "learned" I can apply it to anything I am doing.  I have a couple of scales and one goes to 0.00  weight so that's not an issue...

• the general rule I use is to round percentages of materials to their first two significant figures, and work everything else out from there; the water acts as a variable remainder which serves to make up the formula to 100%
UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
• Thanks Bill!  I spent the last couple of days looking at this formula from every direction and perspective and it seems that I had been following what you just suggested unknowingly.  Adding ingredients early on in this process, I wasn't aware that I needed to adjust the amount of something like a preservative to make it have the right usage rate for a "formula" that was larger than 100% but once I did all the calculations and made all the adjustments, all the numbers fell into place and worked in relation to one another which is also very important in this type of formula.

Now after going through it all and putting everything in the calculator I find that my formula is exactly the same as I had it before which is reassuring!  Adding the little extra .073 type number back into the water phase helped things to work and taking a number like .273 and turning into .3 also worked for me!  Relationships between ingredients and phases are crucial to understand to do everything correctly!  Now I get it!  Thanks guys, for your input!
• You'll sometimes see a formula with the letters 'qs' for water which just means "add enough water to make the formula total equal 100%"

Incidentally, if you are making a formula where 0.75% water would make a difference in the outcome, that's not a particularly robust formula. The amount of ingredients will easily vary from a range of +/- 1% on many production batches.
• Thanks for your reply, Perry!  When you say "if you are making a formula where 0.75% water would make a difference in the outcome, that's not a particularly robust formula." I am not sure I understand.

As far as the production batches, I am working with 100 oz. batches at the moment which , I assume, would be considered small but I always replace any evaporated water so that my total always equals 100% but I can certainly see that in the industry what you stated would be the norm...
• By 'robust' I just mean a good, reproducible, stable formula for large scale production. On large batches (e.g. thousands of Kg) it's not surprising when compounders are off by 1% of water or not. They don't usually add back water. We almost always did it on lab sized batches though.