Means to Calculate the Thrust from the Motor

I found a spreadsheet which calculates nozzle an injector diameters for a desired motor performance (thrust) and permitted combustion chamber pressure. For the life of me, I cannot find where I found it originally, so can't remember who might deserve the credit for the original work. As part of the Scaerotech project, I've taken the formulas from it and reversed the calculations to work any way round. I've been practising my VB skills and created a little application which implements the same equations. Tick two out of the four boxes against the quantities you want to calculate and fill in the other boxes. The results are all calculated as you go along.