If you are using a diazo sensitized emulsion, increased exposure will "bleach" the 'yellowish' diazo and return the emulsion to the color it was before you sensitized it. If it's an SBQ emulsion - there won't be any color change -because there is no diazo.
Nothing points to 7-8. You need to look for extremes - for changes. From your report, there is change between 10 and 11.
Did the 5 minute dissolve? If not, you MAY need to lower your aim so you can find a range where there is color change.
With your experimenting first find where the stencil starts to adhere to the mesh - for you that could be 5 minutes, so 2, or, 3 or 4 minutes will dissolve with water. If that's so- 10 minutes is overkill and wasted energy.
If you're printing water-base ink, you MUST completely cure the stencil so it will survive printing and should be using a dual-cure or diazo sensitized emulsion. If you're printing plastisol, it harmless and won't hurt anything, so it's not so important.
I know you're in your first days, and it's the weekend. When you can, quietly discuss with the person who sold you an exposure unit - Why they forgot to include a US$10 Stouffer T-2115 21 step gray scale that simulates 21 different exposures each time so you could measure exposure. This is a standard photographic darkroom test positive that's been used since the 1930's. They're supposed to know better - you aren't. Once you get one you'll know how many hours you could have saved.
Properly used it should last a lifetime, so I want you to put one on every screen you expose for the rest of your life. It's visual feedback you exposed the stencil properly.
