The highest practical magnification depends largely on what you're observing, your ability to nudge the Dob along, and your determination.
Objects move through the field of view very quickly at high magnifications, especially near the celestial equator, so you need to hone your manual tracking skills.
Seeing is never bad all the time, only most of the time. When looking at sub-arcsecond double stars (which a good 8" scope is perfectly able to split), you will need more than 400x, sometimes a lot more, and also a lot of patience. Views at high magnifications are limited by seeing most of the time, but there is the odd second out of ten (or thirty, or more) every now and then where the view stabilises and reveals what you're after. That counts as a success, once you've seen a double split it cannot be taken away from you
But as I said, this takes skill and perseverance, so for starters you're probably more comfortable with the 150x and lower magnification range.
Cheers
Steffen.
I am so excited now!!!! This is a big step up from my 114 newt I think. so it has a 2 inch focuser so I was wandering if I was to get a eyepiece adapter for my 1.25" eyepieces would these still give as good a view as a 2" eyepiece???
for most people. Highest practical magnification will be anywhere between 150x and 300x most nights and depending on your locality. For most targets, that's plenty of power. In fact, a lot of deep sky objects will require far less as they are quite large. The 8" will show an estimated 90-95 per cent of all objects & features being discussed in the "visual" sections of this forum 
