How to adjust stewing time for different weights of meat?
With lamb (and stews in general) it's not a great idea to adjust cooking times to account for quantity of meat. I have found some useful information on this Chow.com article:


  The key to cooking any tough cut is slow simmering over low heat—lamb shoulder could take upward of two hours to reach the tender zone. Don't be alarmed if the meat seems quite tough after it's cooked for a while, sunshine842 says. The muscle fibers seize up, then relax into a state of tenderness after more cooking. Just keep simmering.


So if you reduce your cooking time to 30% then the meat will have 70% less time to relax and become tender. I suggest to instead stick with the original cooking time and just check it now and then. You can't really over-cook shoulder over low heat.