If you really want to crunch numbers and have dyno sheets and your trans gear ratios, you can do it mathematically. You multiply the rear gear by 1st gear, times that by the torque at whatever rpm you want to check. do the same for second. When the value for second becomes greater than the value for first, that’s the shift point. Pretty simple, but we don’t race calculators, and the real thing is always a little different. 
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93LXcopcarSOLD14.3@96 @ 4500ft 2.02 60ft on street tires.
my 67 ranchero NOT A 390 ANY MORE! 460! 3.70's cast manifolds, comp cams 262H, performer, 750DP 100K out of 79 F250
NEW(oct20/02)14.58@95mph 2.3 60 ft
corrects to:13.86@100
66 merc comet351w, isky roller 600 lift 268/260@.050, vic jr. 700DP, 5000stall, 4.56's c-4, 3400lbs with driver
12.3@110 @ 4000ft 1.69 60 ft
corrects to:11.69@115
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