I just got a FMF powercore and header for my machine along with a K&N Fmf recommends using 150/155 but I'm not sure I ride between 800-1000 ft mostly I'm new to this tuning thing so any info would be great thanks Ry

 

Elev. up to about 1200' is more or less sea level as far as jetting is concerned. Instead of guessing about re-jetting now why not first do a test ride & plug read to find out exactly what your particular engine wants in terms of main jetting? You may find that with your particular pipe & other add-ons you won't even need to re-jet.Some pipe & filter combinations flow more air than others and therefore the only reliable info is that which your own engine gives you.
If you would like to have my tutorial on test rides/ rejetting, you may email me for it.It also has a photo of the plug from my former ride, a Warrior, to show what to strive for.