Tasha, how much of a glutton for punishment (aka, math) are you? If you just want a quick and simple answer, here it is:
- To get the same height/width ratio when doubling a cheese (using twice as much milk - which should give approximately twice as much curds), multiply the diameter of the mold by 1.26
- To get the same height/width ratio when halving a cheese (using half as much milk - approximately half as much curds), multiply the diameter of the mold by .7937
If you are interested in the mathematical formula, which would let you generalize to other ratios, e.g., 3 times as much milk, or 1/4 as much milk, here it is:
[target diameter] = cube-root([change in volume]) * [initial diameter]
Where [target diameter] is the diameter of the mold that you need to use to achieve the intended result; [initial diameter] is the diameter of the mold that gave you the earlier result that you want to duplicate (in terms of height/width ratio); and [change in volume] is the ratio between the desired volume of curds (or milk) to be used over the initial volume of curds (or milk) used.
To illustrate with an example:
You made a cheese with 12 liters of milk using a 200mm mold. (I'm guessing on the diameter of the mold you used - adjust the calculations below as needed for the actual diameter.)
You want to make another 12 liter batch, but you're going to divide it into two 6 liter cheeses, so you want to use half the volume (6/12) of milk (and thus half the volume of curds) per mold. Plug in the numbers:
[target diameter] = cube-root(.5) * 200 = .7937 * 200 = 159
So, you will be looking for a mold with a diameter of approximately 159mm.
If you are REALLY a glutton for punishment, and interested in all the gory details for deriving this formula, here is a link:
http://cheeseforum.org/forum/index.php/topic,14911.msg114054.html#msg114054