when using modav it seems that things can get messy pretty fast with the slightest bug. I need a good fail safe just in case something goes crazy.

Is there a good way to reset the modav value back to default?

Crap I seem to have a knack for figuring something out only after I ask the question....

Care to share so others (like me ) can benefit from your experience?

If you have used ForceAV Stat 0 as your solution I will personally hunt you down and kill you, as I have done for everyone who has used ForceAV in Oblivion, and Fallout 3.
ForceAV is a recipe for mod incompatibility par excellence. Do not use it, ever, unless you're using it on someone who is doing to die soon or who is entirely disposable, as it will screw over any other mods' attempts to use ModAV on the same actor.

For example:
A chem from Mod A does ModAV ResistEnergy +10, waits 30 seconds, does ModAV ResistEnergy -10 for a net 0.
A chem from Mod B does ModAV ResistEnergy +5, waits 20 seconds, then does ForceAV ResistEnergy 0 to remove it for a result 0.
If the player takes Chem A and Chem B, then they'll get ResistEnergy +15, 20 seconds later be on ResistEnergy 0, then 10 seconds after that be on ResistEnergy -10 from that point onwards. Both mods think they're done and so the player has no way to rectify this -10, and can't do it through the console as ModAV works like Restore/DamageAV when called through the console.

To avoid ModAVs getting out of hand, I use

ModAV Stat LastValueSet NewValue to WhateverModAV Stat NewValueset LastValue to -NewValue

This way you always remove your old changes before applying new ones. The first time the script runs LastValue will be 0 so there'll be nothing removed.

yeah the only way to reset mod av is to make sure you keep track of exactly how much you changed and then simply change it back based on that value. Its like walking a tightrope using that thing and one mistake sends you falling with no safety net.

And making a safety net is a pita

avoid modav is the best I can say...