Sure you can use FFG 777 in your 58. The pressure curve will be lower than it would be if you were using FFFG so you will get slower velocities from the same charge measured/volume. Don't be afraid of 30-35gr/vol you won't hurt anything. Hodgedons says to duplicate the velocity of a given BP load, to decrease the charge of 777 by 15%. Lets assume that 777 gives you 20% higher velocities, that would be giving velocities equivalent to a charge of 36-42gr/vol of BP. You will get better accuracy from the lower charge weight though, with larger charges, consistency in loading becomes much more important for good accuracy.