[Lew Silverman]

Following up on what Jim said -

If you check the classification guide for the year, make/model and combination you plan on running, the published factor (for example your 12.77 would be a J class rounded down to 12.50) is the key to determining what "natural" class you fall in. Use the guide to find out what the NHRA horsepower is for your chosen combination (there may be a few for each different engine depending on whether certain engine parts have been superseded) and then use the formula to come up with your minimum weight for that class. You always have the option of adjusting the vehicle weight to run one class higher or one class lower.

Lew