tostig wrote:

That’s a total of 2878 ft-lbs of work. Which means I have 2878 ft-lbs of energy in me for bench pressing.

In my next session I proceeded with normal 10/8/6 pyramid again but couldn’t get past 4reps at 45lbs. How come?

See Oscar's answer. The "tonnage" or energy expenditure has little to nothing to do with the absolute maximum weight you can move. You cannot translate tonnage into any predictions about the max weight you can move, or how many reps you can get any particular weight.

If you want to be able to track your progress and be more confident in predicting what you can do, do this.

1) First, do a max attempt. Warm up with the empty bar, 5 reps. Add 10's, do maybe 3 reps. Go the 25's, do 2 reps. Keep adding weight, doing only 1 rep, until you fail. This is your best guess at your 1RM.

2) Use this formula to predict back and forth how many reps you can do or what your 1RM might be.

Max = Weight * (1 + Reps/33)

or to get how much weight you should be able to do, knowing your max:

Weight = Max / ( 1 + Reps/33 )

or how many reps you should be able to do at a given weight:

Reps = 33 * (Max/Weight - 1)

The formula is not magic. It is not guaranteed. But it is quite useful for trend tracking, if for example you do a set of 6 one day, a set of 5 another, then 3, then 4, and you want to track how those sets compared to each other.

Not everyone likes using formulas like this, but I have found it to be extremely reliable and helpful.