They're very concentrated, so draw off one pint from your FV before bottling and separate a few mls into a shot glass of small measuring cylinder, add one drop of flavouring into the shot glass and stir. Add this dropwise back into pint glass, stirring and tasting between every few drops until it reaches optimum concentration and calculate number of drops needed to dose the rest of batch.
Cheers guys - in the end I used 1 bottle of vanilla extract, straight into the FV, and 3 bags of marshmallows, melted in a little water, added 5 days in. It's all bottled and kegged - I'll let you know what it's like after conditioning, but initial tasting suggested I need a hell of a lot more flavouring...but it its still good.