NL50: set on the turn, how much to bet?

[Specialwon]

This situation comes up from time to time, I rarely have an instinctive figure ready in my mind for how much I should be betting here. Anyone care to suggest a rough and ready guide for bet sizing in this spot without having to think? (there is no time bank on Ongame where I play)

Poker Room skin
No Limit Holdem Ring game
Blinds: $0.25/$0.50
10 players
Converter

Stack sizes:
UTG: $64.05
UTG+1: $48.75
UTG+2: $49.50
MP1: $67.25
MP2: $8.45
MP3: $97.95
CO: $12.75
Hero: $48.50
SB: $82.60
BB: $45.40

Pre-flop: (10 players) Hero is Button with 5[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/spade.gif[/img]
UTG folds, UTG+1 calls, 3 folds, <font color="#cc0000">MP3 raises to $2.5</font>, CO folds, Hero calls, SB folds, BB calls, UTG+1 calls.

Flop: A[img]/images/graemlins/heart.gif[/img] 3[img]/images/graemlins/spade.gif[/img] 4[img]/images/graemlins/heart.gif[/img] ($10.25, 4 players)
BB checks, UTG+1 checks, MP3 checks, Hero checks.

Turn: 5[img]/images/graemlins/club.gif[/img] ($10.25, 4 players)
<font color="#cc0000">BB bets $2.5</font>, UTG+1 folds, MP3 calls, <font color="#cc0000">Hero has another big hand going and gets brainlock</font>.