Take a look at
https://www.riggingdoctor.com/life-aboa ... ting-loads. Harken provides a more exacting formula, although more complicated.
Using that formula, we're looking at about 151.8 pounds of pressure at 20 mph wind speed on the main sail. At 30 mph wind speed, that pressure increases to 341 pounds. Those numbers would translate to about 57 pounds of pressure on the main sheet in your hand, a number that seems consistent with the pressure I'm pulling in those conditions by feel. (Think about picking up a 5 gallon pail of liquid. Weight is approximately 45 pounds.) At 10 mph wind speed, the pressure is down to less than 40 pounds.
Actual numbers would need a strain gauge just before the boom. I doubt it would be worth the cost and difficulty to actually measure this.
If you're just curious, this should be enough to get you going. If you're breaking something, you need to say what part.
Jim Clark-Dawe