hmmmm.. i have firebolt 5 so... if i charge four at a time, it will be 5x4=20, so do you have the wand out until your mana pool reaches 0, and then do the switcharoo?? or do you just let all your initial mana go to waste in order to save dura??
If you charge 5 at a time, that's 6mp/charge * 5 = 30 mp. Some fluctuation due to mp penalties, but about 30 mp. However, you only need the base amount of 6 in order to start charging fb, so a single mp10 gets you 5 fb charges in 1.5* seconds. You now have 0 mp, so it doesn't matter if you tuck your wand away.
*edit: 2 seconds