Re: Low Tech cannons mortars and shells
 

I am also very dubious of using the RoF rules to handle many projectiles vs. many targets in an area of effect. They were designed for shotgun blasts and machine-gun bursts against individual targets. Instead, I would start with empirical data or back-of-the-envelope calculations about hit changes at various distances down the cone, and pick numbers such that chances to hit are in the right ballpark.

Re: Low Tech cannons mortars and shells
 

The basic scaling should be that, say, 1 projectile per square yard is a fixed hit chance. Using the range/speed chart, you can get that by looking up (SM for projectile count) - (2x SM of area) (at 0, that is 1 projectile per 2 square yards), which gives us roughly:
+6: expected hits 3, hit chance 95% (15-)
+5: expected hits 2, hit chance 87% (14-)
+4: expected hits 1.5, hit chance 78% (12-)
+3: expected hits 1, hit chance 64% (11-)
+2: expected hits 0.7, hit chance 50% (10-)
+1: expected hits 0.45, hit chance 37% (9-)
+0: expected hits 0.3, hit chance 26% (8-)
-1: expected hits 0.2, hit chance 18% (7-)
-2: expected hits 0.15, hit chance 14% (7-)
-3: expected hits 0.1, hit chance 10% (6-)
-4: expected hits 0.07, hit chance 7% (6-)
-5: expected hits 0.045, hit chance 4% (5-)
-6: expected hits 0.03, hit chance 3% (5-)
-7: expected hits 0.02, hit chance 2% (4-)
-11: expected hits 0.005, hit chance 0.5% (3-)

8+(SM for shots fired) - (2x SM of area) does a decent job over most of the interesting range