I pull 55# on a Whisperstik Mojo and use a 340 shaft to get 29-31% FOC.400 spine would get me in the upper teens,maybe 18-19%.I think,and this is only a wild guess,that 340's might give you FOC in the mid 20's range.I would start with a full length shaft,100 gr insert and 250 gr point and be prepared to go heavier or lighter if you need to.You can use different weight steel adapters and glue on field points to customize weights if you need to.I think 400's would get you in the normal range of FOC.