Power Items

An Alternate Option, instead of the complexity of Powerstone:

A spellcaster can designate 1 item they own as their Power Item, it holds extra FP for casting spells, and when it is out, it must be recharged at a temple, guild or whatever the DM says for the cost of 5 per FP. 
Round up the Cost of the item being designated as the Power Item to the bracket it fits into. Eg. A staff that costs 10 still holds 1 FP.

Cost FP
50 1
120 2
220 3
340 4
490 5
670 6
880 7
1,100 8
1,400 9
1,700 10
2,000 11
2,400 12
2,800 13
3,300 14
3,800 15
4,300 16
4,900 17
5,500 18
6,200 19
7,000 20
12,000 25
18,000 30
26,000 35
37,000 40