measuring preboil gravity ??

I am in the process of building up my brewery and many variables are changing. I cannot rely on previous efficiency numbers to hit my OG. This makes finding out the preboil grav that much more important. Many people talk about not being so concerned with preboil volume but very concerned with preboil grav. When using a hydrometer how are people taking several samples to establish when to stop sparging? or do I have this all wrong?
Thanks in advance
Dave

Get a refractometer. You can sample as often as you like, just a few drops is all you need.

Pro brewers use refractometers and stop their sparges when the worts hit a specific plato.

I agree with those above but to me I want to end up with a certain volume at a certain gravity. I want to boil for 90 minutes every time and hit my numbers. Since your system is changing you're going to have to relearn it.

If your kettle has changed then you need to find out how much you boil off in the time you use for a boil.

I don't get concerned with gravity on the sparge water as we pre treat it to a ph of around 5.4.

We know how much we need in our kettle at the beginning and by the end of our boil we will be at the correct volume and gravity.

Adjust your recipes to match your boil times instead of over or under boiling.
It sounds like your old system was dialed in. Just need to do the same with the new one!