Just had flash tune done on my 4speed ford ba with 4:1:1s came up with 152rwkw what is it in kws thx

Maybe 210kw. Someone else would know the maths to work it out right

There is no standard formula; every setup is different. Drivetrain losses can range from less than 10% (eg, a purpose-built race car with manual transmission and light flywheel) to more than 25% (eg, an automatic with high-stall convertor). Even diff oil viscosity, wheel weight and tyre pressure can make a difference.

For an automatic in a road car, you would be lucky to get better than 80% efficiency (20% loss). Using this as a rule of thumb, 152 rwkw means you have at least 190 kw at the crankshaft (152 / .80), possibly 200 kw. I haven't seen any back-to-back engine and chassis dyno runs on a Falcon six, so this is just an educated guess.

approx. 200kw at engine

While on this subject, are drivetrain losses a percentage or an offset? Like a 1000hp engine would make 800rwhp, but what if there was a 200hp engine, it would have 160rwhp? or 0? Why does the 1000hp engine lose 200hp?

Based on the numbers I have seen, it is somewhere between a constant percentage and a fixed offset, but closer to a percentage. All other things being equal, the percentage of drivetrain loss will decrease slightly as power increases. So, a 200kw engine might lose 20%, while doubling it to 400kw with the same drivetrain (assuming it could handle the power) you might lose 17-18%. It's that sort of relationship.

I'm sure someone, somewhere has studied this relationship and described it scientifically, but I haven't come across it. In any case, the only power you can use is what gets to the road surface, so engine kw becomes mostly academic.