I always have trouble when I get a contract to place a satellite in <orbit x>. How much dV am I gonna need to get there?

Here's the answer: Your orbital velocity, for a circular orbit, is approximately 2/sqrt(H .6) where H is height above surface in Mm (10⁶ m) and the velocity is in km/s (just multiply by 1000 to get m/s).

To be specific, a more accurate approximation is 0.94 * 2/sqrt(H .6). Ie, the approximation I list above over-approximates by about 6%

So, if you're starting from a 100 km orbit (velocity = 2/sqrt(.1 .6) = 2/sqrt(.7) = 2.246) and need to get to a 6Mm orbit (2/sqrt(6.6) = .7785) You'll need 2.246-.7785 = 1.4675 km/s = 1467.5 m/s to change orbits.

And if you want to be more precise, remember you can factor: .94*v2 - .94*v1 = .94*(v2-v1) = .94 * approximated dv above. So using the same example, we know we'd really "only" need 1379.45 dv. But, tbh, I like having the 5% wiggle room to account for cosine losses around burns, and other issues that may come up during a launch.

Furthermore, H .6 is actually your radial distance from the center of Kerbin, since the height to surface is .6Mm. So really, the velocity is 2/sqrt(r) (or more accurately, 1.88/sqrt(r) )