What is the best way to reduce a draft when the draft calls for a loom that is bigger than the one you have at home?
I assume you are asking how to alter the project directions to weave something narrower than the stated width to fit a narrower loom, not how to reduce a draft to fewer shafts. The former is easier to do than the latter!
Let's say you are looking at a draft and weaving instructions such as those for Connie Childs Elliott's Huichol Towels (Handwoven, January/February 2011, pages 56–58). The width of the towels in the reed is 18-1/4" and let's say your loom has a weaving width of 16". The sett for these towels is 24 ends per inch, so you need reduce the number of warp threads from 438 (18-1/4 x 24) to 384 ends (16 x 24). You'll need to take away 54 threads.
Now look at the draft and warp color order. Both the 21x repeat in the color order and the 27x repeat in the draft contain 16 warp threads. If you reduce both of those repeats by 3x, you'll be eliminating 48 threads (6 fewer than you need); if you reduce them by 4x, you'll be eliminating 64 threads (10 more than you need; your weaving width if you did that would be 374 threads divided by 24 = 15-5/8".
In looking at the draft, however, I see a straight progression in the threading of those four blocks throughout. It would not hurt the structure to add 8 threads to the 374 (threading them 3-4-3-4, 5-6-5-6 at the end. So that the colors work, I'd add 4 navy threads to each side. So, the new draft and color orders would look like this.
In summary, the steps to follow to make a draft narrower (or wider) are:
Unfortunately, most solutions involve a little math.
Hope this helps!