Question about velocity from flat ground

I was playing catch with my dad earlier from about 60 ft. or so, pretty close to the regular distance. My dad has a little radar thing on his glove, so I was trying to see what I could get clocked at just throwing normally (not pitching, just playing normal catch from flat ground). I was in the 76-82 range. So my question is, is there any way to judge how fast I throw from an actual mound pitching given that speed from flat ground? Does it normally stay the same, or do u gain 3-4 (or whatever) MPH more?

It depends if you can pitch downhill and use the mound to your advantage.

At those speeds from 60 ft. the glove radar tends to read about 5 to 6 mph lower than a JUGS radar gun. A lot depends on how the pitch is caught though with the glove radar. Typically, you’ll see a 1 or 2 mph increase when throwing off the mound with the same level of effort. That isn’t always the case, but it usually is.

I know the literature says to use 1 mph/7ft but the reality is that the radar gun isn’t instantaneous so there’s a little drop in speed due to that, the ball doesn’t slow down as much for slower pitches as can be derived from looking at gameday fastballs from the slower throwing pitchers vs. the faster throwing pitchers, the pitch isn’t delivered from a full 60’ 6" and the glove radar picks the ball up a few feet out in front of the glove.