If you have seen the second one of these that you mention (the one with a positive MPC on top of the fraction), I do not believe it could possibly be correct. The reason for this is that the tax multiplier must always be negative.
Think about it -- if taxes go up, do we get more aggregate demand? No -- we get less. So if taxes go up, the change in AD must be negative.
When we use the tax multiplier, what we are saying is this:
Change in taxes times multiplier = change in AD.
So a positive change in taxes (they go up) has to equal a negative change in AD. Mathematically, that means the multiplier must be negative.
If you have a positive MPC on the top of your fraction, the multiplier will be positive and that can't be.