Honestly, I would switch to a fast acting insulin and a separate long lasting insulin instead of Humulin. Humulin M3 is a mix of fast acting and long lasting together. It has some weird peaks with it too. But if you change your dose, say you raise it because you are running higher blood sugars, you run the risk of having too much long lasting insulin in your system too.
It becomes tricky. If you take it every 12 hours, and you stayed high 12 hours, feasibly you take a couple units extra to try to come down........... but then you also have the extra long lasting for about 12 hours after with the weird peak of the short acting.
That is one reason it's not popular anymore. If you have them separate you take a basal insulin at the same rate daily then you can vary your fast acting one to meet your needs. Much nicer.