[Ajax_MD]

A slack rig is far more damaging than a tight one. Movement and shock loading hurts, constant (proper) tension does not. For all the money people spend on boats, a Loos gauge is really not expensive. Buy one, follow the directions, and do it correctly.

This.
Shock loading is bad.

I required 15% on a Loos Gauge to get shrouds so that they were not sloppy on the leeward side. Slightly soft is acceptable. If you achieved that with 12%, then good for you.

I'm not sure if your rigger educated you as he worked, or if he just did the job and left, but the forward lower shrouds should be tensioned to put a slight pre-bend in the mast. "Telephone pole" rigs like yours and mine will only achieve a slight amount of pre-bend.

The lower shrouds will and should be more slack than all the rest. Their primary function is to check mast "pumping" when sailing, or when the wind blows across the spar just the right way.

If you knew this already, I apologize.

 

[Ajax_MD]

CC-

Do not expect things to remain exactly the same under sail, as they are at the dock. There are no loads on the rigging at the dock, and things do move and bend.

You don't need to sight up all the way from the base of the mast. Plant your cheek on the sail track above the boom, and sight the masthead.

If the mast tip is falling to leeward, this is bad. The upper shrouds need to be adjusted.
If the center of the mast is falling off to leeward, the lower shrouds need to be adjusted.

You could also try a laser pointer sighted along the track, pointed at the masthead. I imagine that you'd have to try that early in the morning, or in the evening, when ambient light is lower.