The specific tanks you have to are going to change what your optimal ratio is, but the general schools of thought on that tend between 'throw a wall of medium tanks at it' and 'throw a heavy at it and then walk artillery up behind that'. General advice, try to get a T-70 if you don't already have a PT-76. That area armor boost is massively important to your survival in some fights.
I kinda want both so I can chain defensive actives. . . welp, I've got an entire year to hoard cores for Eliza now.