Calculating the output is quite easy.

Estimate the distance to the subject in meters (e.g. 5 meters). If you not sure, you can focus first then read the focus distance off the marking on the lens.

Multiply the distance by the aperture f-number (e.g. 3.5), so 5 x 3.5 = 17.5

Take the guide number of the flash (38 for the YN460), multiply by the square root of your ISO (e.g. 400) divided by 100. So sq root of 400/100 = sqrt(4) = 2, then 38 x 2 = 76.

Now divide 76 by 17.5... 76/17.5 = 4.34 Take this number and set it to the nearest flash power ratio, i.e. 1/4

Once you get the hang of it, you can work it all out very fast in your mind, faster than using one of the more expensive P-TTL lenses.

Or not