For a 1 in 48 twist, which is most likely what you have, you should look at conicals up to around 400 grains (for a 50 cal) and a max length of around .750". This is not a hard rule but gives the approximate optimum for that twist.
What would be the max length for a 54 Caliber Hawken with a 1 in 48" twist and a 28 inch barrel?