[many birds]

Trying to tune the ignition on my setup. My distributor is an aftermarket HEI—Pertronix. Purchased by previous owner. I have already changed the module for a GM version and welded a limiter on the vac can. The vac is limited to about 12 degrees. However, my mechanical advance only adds 10 degrees at the crank. What’s going on here? And how do I fix it to get the recommended 20+ degrees?

Thanks!