Gaining a greater health from Potions?

Here are the two subroutines in bank C3 that calculate the amount of hp/mp restored when you use an item from the menu. funkydiscogod has the right idea, you could modify these if you want to change the effect in the menu.

Code:

; [ Load Item HP/MP Restored ]
; this is for items that restore an exact amount of hp/mp

C3/8CCD: BF 14 50 D8  LDA $D85014,X   ; hp/mp restored
C3/8CD1: 85 B2        STA $B2
C3/8CD3: 64 B3        STZ $B3
C3/8CD5: 60           RTS

; [ Calculate Fraction of Max HP/MP Restored ]
; this is for items that restore 1/16ths of max hp/mp

C3/8CD6: AD 12 42     LDA $4212
C3/8CD9: 29 40        AND #$40
C3/8CDB: F0 F9        BEQ $8CD6
C3/8CDD: A5 F3        LDA $F3
C3/8CDF: 8D 1B 21     STA $211B
C3/8CE2: A5 F4        LDA $F4
C3/8CE4: 8D 1B 21     STA $211B
C3/8CE7: A5 B2        LDA $B2
C3/8CE9: 8D 1C 21     STA $211C
C3/8CEC: 8D 1C 21     STA $211C
C3/8CEF: AD 36 21     LDA $2136
C3/8CF2: 85 EB        STA $EB
C3/8CF4: 64 EC        STZ $EC
C3/8CF6: C2 20        REP #$20
C3/8CF8: AD 34 21     LDA $2134
C3/8CFB: 85 E9        STA $E9
C3/8CFD: 46 EB        LSR $EB
C3/8CFF: 66 E9        ROR $E9
C3/8D01: 46 EB        LSR $EB
C3/8D03: 66 E9        ROR $E9
C3/8D05: 46 EB        LSR $EB
C3/8D07: 66 E9        ROR $E9
C3/8D09: 46 EB        LSR $EB
C3/8D0B: 66 E9        ROR $E9
C3/8D0D: E2 20        SEP #$20
C3/8D0F: 60           RTS

One idea that might work is to add something like this in an unused part of bank C3 (maybe somewhere in C3/F091-C3/FFFF) and then replace the address for the original hp/mp subroutine (at C3/8C41) with wherever you add the new code. This will multiply an item's hp/mp value by 40.

Code:

LDA $D85014,X
STA $4202
LDA #$28        ; multiply by 40
STA $4203
NOP
NOP
NOP
NOP
LDY $4216
STY $B2
RTS

C3/8C40: 20 CD 8C     JSR $8CCD       ; change this to point to your new subroutine