The problem with that is: Different ships have different needs for shield-sharing. For example, a ship with a few turrets its own size (and comparable shields) requires little to no shield sharing, as both parts are equally capable of resisting damage. A large with many small turrets would want a high shield sharing, as the smaller turrets are significantly more vulnerable to getting destroyed before the battle ends.
Let's look at the factors that determine how much the shields should be shared:
Absolute size: smaller turrets should get more mothership shields.
Number of turrets: more turrets less mothership shields. But with fixed gun emplacements also able to do damage, what counts as a turret?
Damage done per shot: more damage hitting the turret should mean more mothership shields (See point about nuking off small turrets)
Mothership shield levels: the less of its shields the mothership has, the more damage the turrets should take.
The big can of worms is the number of turrets. Since obviously a larger ship should be able to handle more turrets, but also whether a turret is large or small mattering. Maybe the AI revamp will include a few metrics that would be helpful.