Even better if you have electronic scale:

Place treacle tin on scale, switch on [scale will show zero weight], use spoon or spatula [down boy] to remove some treacle. Scale shows as a minus figure the amount taken, if less than required dump some treacle off the spoon into other ingredients then repeat until your latest spoonfull takes the amount taken over the target. Hold spoon above the tin until the trickle-back shows that you have removed the right amount, clean spoon by stiring treacle into other ingredients.

No mess, no calculation, no possibility of having to try and take treacle back out of the dry ingredients.

On a spring scale you can use the same movements, but you will have to do the subtraction calculation yourself.