I have a number 12 copper coated wire from house it gose 300 feet the panel in house supplies 120 volts it is to power a 1200 watt saw what would be my approximete voltage to the saw

"Coated?" What type cable are you using. If you are using #12 AWG, type UF, then you will get 96 volts at the receptacle.

If you are going +/- 300 ft. then you should be using a #4/2, Type copper THHW.

Be aware that there is a requirement that the end to end Percentage of Voltage Drop of 5%

Line loss on 12 gauge wire at 300' would be about 14 volts. About 11.5% drop.
So you will get about 106 volts to your saw.

Be aware that there is a requirement that the end to end Percentage of Voltage Drop of 5%Point of clarification.
There is an NEC suggestion to keep VD to a tolerable level. It is not an NEC requirement.