A lot of the old LeBlond Regals (at least the "roundhead" series) had a handwheel on the driveshaft, projecting out of the belt guard. LeBlond equipped the Roundhead Regal lathes with this handwheel to encourage operators to manually roll the headstock gearing to get the gears into proper mesh before starting the motor drive. If the "lumphead" Regal- which I suspect the lathe in this thread is- has the handwheel, the matter is even easier. No belt guard to remove. Make a chalk or paint marker mark on the handwheel, and make a similar mark on the chuck, each at 12:00. Put the gear levers in one position, roll the handwheel, and count the turns to get the chuck to make one turn. Chances are it will be a few full turns + some fraction of a turn. Round off the fraction of a turn to the nearest quarter turn, and calculate the gear ratio. If you know the pulley sizes on the motor and headstock drive shaft pulley, and the motor rpm, you can do the math for that ratio. Multiply the motor rpm x (small motor pulley diameter/large headstock drive pulley diameter). This gives you your input shaft rpm. It is a constant. Multiply the input shaft rpm by the gear ratios for each setting of the gear change levers on the headstock.
This is basic ratios, stuff that was taught to us back in grade school. With today's pocket calculators, it should not be too much of a job to run the numbers.