Im not totally sure what you mean? If your talking spirits you just need to know if your glass is a single or a double. Sometimes pubs serve doubles as standards (for the £££) so its best to ask, or just specifically ask for a single if you're not sure, although again some pubs only have a double optic on the bottle. Also depends what optic your pub is using as a single can be 25ml or 35ml (so a double is 50ml or 70ml).
The syns book gives them per 35ml, thats probably better so you dont underestimate. All spirits are between 4 and 5.5 per single measure, heres the main ones:
4 syns
Bacardi
Brandy
Gin
Malibu
Rum
Vodka
Whisky
4.5 syns
Jack Daniels
Peach schnapps
5.5 syns
Southern Comfort
Tequila
Basically I would stick to one pub and check/ask what size optic they are using (it has the ml written on the front so you should be able to see without asking anyway, uless they use a thimble in which case you can just ask for a single anyhow) and stick to singles if you can. Obviously have it with diet mixer.