Scottski,
That's a good start, having a spine machine but there are so many variables you have to consider. What type of handle design, what point weight, finished arrow length and the list goes on and on. If your 43# at 28" with a non center shot bow using 125grn points with a 29'' arrow 40 to 45 spine might be a good start. The great thing about your spine machine is once you find what works you can copy it every time. 35 to 40 may be the magic number...Good luck and go at it. I always spine and here's one I made off a website sometime ago