How far should I put a 300watt lamp away from the plant?

You can get a luxmetre apps on any smart phone. Not the best way to mesure light but its the short/beginner freindly one. Do some reading about PAR/PPF/PPFD to get some advanced knowledge. Guide line for lux : seedling and clone 5000-7000 Veg : 15000 50000 Flow 45000-65000 absolute max is 75 000

It depends on your light and normaly you should have a manual where the company shows you the output on which distant and radius your light emits the amount of PAR that is needed. In your local hardware store you can find cheap tools ( 5 to 10 bugs Heaviesk Outdoor Hygrometer Soil PH Meter Moisture Meter 3 in 1 pH Detector Analyzer Pointer Base for Garden Operation Flower Floor ) to measure the amount of lights that hit your plant. My own 300 watt light manual said something about 24 inches above the plant and the center but this is just in an 1" square if I remember it correctly and decrease very fast further away form it. So go and buy this little tool it will also help you also to check out the water in your soil to avoid over and underwatering. Also if I´m remember it right the plant grows best in a range of 700 till 1000 PAR ( the amount of photons per sec² per meter what ever for more growth over the 1000 you must add more CO² ect). But don´t forget to check the tempreture if you have a weak light that must be very close to the plant to reach that amount of photons.

Around 18-24 inches mate, start at 24 then gradually lower it closer as long as there is no sign of stress