Don is quite right; even under the best of conditions, you're really pushing it when you try to run a "230v" motor on 208 volts.
As a general rule of thumb, a motor may be used at plus or minus 10% of the nameplate marked voltage. So if your nameplate avtually reads simply "230v", then it should have a minimum of 207 volts at all times- and make sure that value is measured "under load!"
Now, if the motor is marked 220, or 208/230, etc...well, you get the picture.
I would also be very conservative in allowing for voltage drop. Remember, motors- especially those used in compressors- have a considerably higher curent required for starting; you almost want to base your voltage-drop calculations on the "LRA", or 'locked rotor amps.'
An exhaust fan isn't really fighting a load as it is starting up; so the inrush current needed isn't as great as it might be in other applications. You will probably be able to use #8.
One thing to also consider is a motor satrter with electronic overloads ("heaters"). Even though strictly not necessary, it will provide additional protection for the motor. The electronic elements provide protection against many other power problems than simple 'heaters' do.