I can't say for sure what an acceptable engine temp is, but I do have some bitter incite into what an un-acceptable temp is --
Bear in mind that all gauges in cars are not necessarily calibrated correctly, but this is my experience. My TD 356 with it's rear mounted radiator did not ever cool the way I would have liked. It showed between 200 and 220F on it's brand new VDO gauge every day. No matter what I did to the air scoop, ducting, or fan, it just would not run any cooler. After a while, I came to believe that this was a "normal" temp, and decided not to worry about it. Then one fateful day I took it for a long interstate run on a day of 85F temps. This time the gauge went to 230F, and then wouldn't come back down --
By the time I got home it was smoking, and the next day it ran V. badly, then it somehow (I think the cam bearings seized) jumped time and destroyed the head -- the pistons said hello to the valves and broke the cam into 5 pieces, etc. It also sheared the key from the timing pully, making it appear to me as if the crank was broken. Worse, that was the day we were to load the car on the trailer for the trip to Newfoundland to compete in the Targa rally

It's back in the shop now being fitted with dual motorcycle rads mounted in front of the front wheels, and I assure you that I will not stop development this time until it runs at temps less than 210F (on my gauge) in all conditions.