Bakers Percentage for Starter

It depends on your timing and temperature, but as a rule I figure it takes 12 hrs to grow a bit of starter up to 20X the original weight if it is at 100% hydration and somewhat longer if the hydration is lower.  Of course everybody has a different kitchen temperature which varies over the year so your mileage may vary as they say.

To get the 15.5 oz of starter that you want you will need 0.75 oz of seed starter, 6. oz of water and 9.5 oz of flour plus some for the bowl and the scraper.  These are awfully small quantities to be measuring so, since it is going to be quite a stiff starter you might mix up a bit extra, knead it to combine, then cut it down to 16 oz or so to ferment.

Personally I have never seen that much difference between a batch made with a stiff starter and one made with a 100% hydration starter.  And with a 100% hydration starter you always know that there are equal part of flour and water in whatever amount you use, so calculations are quickly doable in your head.  And if you have to train your friends to calculate baker's percentages just to try your recipe, there are likely to be few who actually carry through and make it (which might be OK depending on your intent).

For a 1:10:10 expansion to yield 15.5 oz, the initial inoculation needs to be 1/21 of the final weight.  You are doing a 1:12.19:7.80 expansion and 20/(1+.64)=12.19 and 20*.64/(1+.64)=7.80 so you need 1/(1+20/(1+.64) + 20*.64/(1+.64)) of inoculant (note that this is 1/21, the same as it is for a 1:10:10 expansion).  The other 20/21 is (within rounding error) in the 12.19 of flour and the 7.80 of water.

The other way to get it is to divide the 15.5 by 10 (to get 1.55) and then by 2 (to get 0.75+) and call 0.75 close enough since you can't measure sticky stuff very well anyway.

See this post on the 1,2,3 approach which works very well and is very straight forward.  Essentially the recipe is divided into 6 parts, the first being the ripe starter (100% hydration starter) which is 1/6th or 16.67% of the recipe- a very sound percentage to jumpstart a nice rising loaf.  After adding stage 2 (33%) your total is now 50%, the last stage adding the remaining 50%. 

So making two loaves of 2 pound each is 907 grams (metric is always easier, buy a scale for $30 that goes up to 6 pounds and work in grams).  Your stage 1 starter would be built to 151 grams or about 5.3 oz. That means you would have built up from a couple of tablespoons and have a peak starter at this point plus a little extra that you will use to store until the next bake.  The second stage would be double that or 302 grams, or 453 total at this point.  Last stage would be remaining giving 907 total when finished.  So an easy way and readily scalable.  There are recipes posted using this method too.  See this post.

http://www.thefreshloaf.com/node/9346/123-easy-formula-sourdough-bread

Type  1,2,3 into the search box above...

At least when it comes to breadbaking.  Especially if you use a spreadsheet to tweak formulas.  And moreover, size.  A recipe for bread dough can easily be tweaked to be pizza dough in terms of hydration, and flour composition.  I may want 3 pizza doughs at 400 grams each, or I may want two batards at 500 grams each.  It gets real easy to change the size or hydration percentage going this route.  Lots of free spreadsheets out there and posted at TFL.  For all else?  I use ounces/pounds!  Good luck

See these BBGA standards here: http://www.bbga.org/files//2009FormulaFormattingSPREADS.pdf 

Although nothing posted above is wrong in any way, this is a concise and complete explanation of Baker's math and formula formatting.

You do not need to switch to grams to use baker's math - but you may have to find a scale (and there are such scales and I use one) that show ounces in decimals rather than fractions. 

Have fun!