I haven't played RQ in ages, glad to have a chance to get back into it.
Tap was always a rather limited availability spell. Some considered it evil, others as extremely bad taste. The rest tended to think of it as something to keep to themselves, just incase an apprentice decided to get a magic library the easy way...
First step to control Tap, just make those spells extremely rare. And if a few mage-slayers come hunting folks who are known to use it, then so be it.
That sucker has got to drain more than 1 point, otherwise it's just a null gain. In which case you might as well use a diminish spell.
One of the opinions on this is easy. Magnitude == Charactaristic yields 1drain/1mp gained (net nothing). Each Magnitude > Characteristic yields +1drain/+1mp gained. This is like others have said. Each point of magnitude greater than the target characteristic increases the permanent drain by 1 and the mp gain by 1.
Alternate Form. After thinking about how resistences work, that can be used for this if you think about it. Here is a really sketchy shorthand version of it. Make an effect table like Dodge and Parry. Failures, obvious result. With the normal success being the difference between casters spell check and the defenders lower(but not critical) persistence check. Just look that number up on the Sorcery Manipulation table as a magnitude, that's how much the defender looses (not to exceed magnitude or drain past one, of course).
For example, Evil Fred spends his mp and casts Tap succussfully with a roll of 73, while Roadkill George rolls a successful persistence check of only 40. Evil Fred wins by a margin of 33 (73-40=33). Looking on the chart, that yeilds an effective magnitude of 4. So poor Roadkill George looses 4 points while Evil Fred gains 4mp.
That form is a little more complicated, but it has variable results. You may even limit the maximum effect to not exceed 1 greater than the magnitude of the spell minus the characteristic targeted.
Until we get an official ruling, this is going to be rather contraversial. I'd heavily suggest restricting access to that spell. Let's face it, it's either a screw yourself spell, or torture prisoners spell.