or can someone tell me a way of figuring this myself?
The only way to know the exact spine needed, is to test the set-up. Start out with arrows that are slightly weak according to the mfg. Leave them long and set your bow draw weight to it's mid range. Then bareshaft test and adjust arrow length, tip weight and/or draw weight to fine tune exact dynamic spine. Using this method, you can get a huge range of arrows to shoot properly out of a bow.