I have read others say you should have at least 2 inches from the 9 inch mark, I guess?
This is mainly for water, but also applicable for air as well. Explanation will in the below section.
I started originally at the 7 inch mark, now I can get to the 8 inch mark. Should I get the longer cylinder, because now I’m less than two inches from the top?
If you have the 9in cylinder, you can still use it until you max out. That includes your current penile length hitting the 8in mark in the cylinder when under pressure. This is assuming you haven't maxed out the cylinder's wall yet. If you still have space around it, great. If you already maxed out the cylinder wall after a pump session, all your vacuum force will pull your penile body upwards, maxing out the top as well. With air, it's not a problem. With water, you want to have cushion space around the penile shaft and the top. This cushion of water allows the penile body to equalize the internal and external pressures to properly create expansion that truly optimize your growth. Air creates hot spots that will create uncertainty in the expansion throughout the penis. Water, not so much since what cannot be compressed, and it minimize the hot spot creations. As a result, your penis expand equally across the shaft, especially the internal areas.
The 2in of top space is to create that vacuum equalization, as well as generate the air pocket elimination buffer zone. Too small of a pocket at the top reduced the chance where air bubbles can accumulate for elimination. We tend to think the lesser the space, the lesser the needs through logical thinking. In physics, the more the space, the lesser the work.
My next question would be, why can’t I keep using the 9 inch until I get there? What is the science behind having more than two inches from the top? I just can’t wrap my head around the reasoning.
You can. Use it up until you have nothing left at the top, with air that is. For water, when you only have 0.5in left at the top, you also need space around the penis to free as well for pressure equalization. Without space at the top and around the penis, you will not get that equalization benefit for proper internal expansion.
A lot more if you need it.