It has just occured to me that the effects of skill adders have not been considered here. Since skill adders only increase the mastery bonus and not synergies, taking them into account will in fact decrease the level at which synergies become useful. With this in mind, we can calculate at what level of mastery an investment in synergies will be more beneficial than another level of mastery. The results for some popular spells are:
Meteor: 6 (assuming 1 point in Fire Bolt, Fire Ball, and Inferno already)
Hydra: 18 (assuming 1 point in Fire Bolt and Fire Ball already)
Chain Lightning: 16 (assuming 1 point in Charged Bolt and Lightning already)
Lightning: 3 or 4 (assuming 1 point in Charged Bolt and Chain Lightning already)
What this mean is, for a spell such as Hydra, you should invest in Fire Mastery until it is at L18 with skill adders included, then alternate between one additional level of synergies and one additional level of mastery. If your Sorceress had a +10 skill setup, this would imply she should start investing in synergies for Hydra as early as L8 Fire Mastery.
If the number of skill adders "X" you have exceeds a break point "B", then you should invest "X-B+2" points in synergies before you begin increasing masteries. Given +10 skills with Meteor for example, you would want to toss 10-6+2=6 points into synergies before you start raising Fire Mastery.
Edit: Made a small calculation error so I have corrected the break points.
Meteor: 6 (assuming 1 point in Fire Bolt, Fire Ball, and Inferno already)
Hydra: 18 (assuming 1 point in Fire Bolt and Fire Ball already)
Chain Lightning: 16 (assuming 1 point in Charged Bolt and Lightning already)
Lightning: 3 or 4 (assuming 1 point in Charged Bolt and Chain Lightning already)
What this mean is, for a spell such as Hydra, you should invest in Fire Mastery until it is at L18 with skill adders included, then alternate between one additional level of synergies and one additional level of mastery. If your Sorceress had a +10 skill setup, this would imply she should start investing in synergies for Hydra as early as L8 Fire Mastery.
If the number of skill adders "X" you have exceeds a break point "B", then you should invest "X-B+2" points in synergies before you begin increasing masteries. Given +10 skills with Meteor for example, you would want to toss 10-6+2=6 points into synergies before you start raising Fire Mastery.
Edit: Made a small calculation error so I have corrected the break points.