BobW is spot-on the money.
My go-to for some 1/2" x 2" LOC variable-flute endmills was always 6100RPM, 100"/min, and .050" WOC. (I never bothered to calculate the speed/feed, because these lazy-man's numbers worked well-enough for my onesie-twosie jobs, with my machine & holder combo --- Nikken milling-chucks for-the-win BTW...) On something that long though, I'd do exactly what BobW suggested, and break it into (2) depth passes, and just like he suggested, I'd leave an extra .005"-ish stock on the second/deeper pass, so that I didn't rub the shank on the walls. It always worked like a charm.
Even with a 4-flute tool, yes, I'd still take that small a width-of-cut. Even though people assume that 4-flutes = wider cuts, the problem - like BobW already mentioned - is the length-diameter ratio. The flute-space removes a lot of stability from the endmill, and thus, you have to reduce the width/radial-load to keep the cutter from deflecting away from the cut.
The better solution, is to either use a shorter cut-length endmill with a long neck, or use an endmill with a smaller gullets (flute-space) which is stiffer, and still take a small width of cut.