What measurements from the rear wall?

Different types of speakers = different distances. In my experience panel type speakers will work best at 5+feet where you can expect greater clarity due to increased time in the signal bounce from the wall behind the speakers.  Ironically, due to their dispersion pattern you can usually place them closer to the side walls than box speakers (which in turn you can place closer to the wall behind them than the panels without losing as much clarity). It is hugely helpful to understand the existence of, and location of, bass nodes (using a SPL meter and source of LF sound) so you can avoid placing your speakers in a bass node (and your listening chair as well).

Oh, BTW, measure from the speakers front baffle as suggested above.

Treating the wall behind the speakers appropriately can reduce the need somewhat if needed. 

Any recommendation on placement is just a rough guide.  The specific location that works the best can only be determined by trial.  You can jst try moving the speaker on a somewhat random basis, and in time you will find a good location (this can take quite a long time).  Or, you can employ certain methodologies for setting up speakers.  The methods that work relatively well include the Wilson method, which emphasizes getting the midrange to sound natural, or the Sumiko method, which focuses on bass response. 

I've used the Sumiko method to help set up a number of systems and it works quite well.  What is surprising is that bass response can change quite a bit with movement as small as an inch or two.  There are many suitable locations that one can find with this method, so one is not trapped into a location that does not work for whatever reason, but, on the other hand, this makes choosing not as simple.  What is remarkable is that it is not always the case that closer to the room boundaries, the stronger the bass; there are many nodes such that strong and reasonably even bass can be found in locations well away from the front and side walls (a good thing because imaging is often best with speakers well away from room boundaries).

Go to one of the online room mode calculators, and put in your room dimensions. The calculator display will show where in the room are located all the room’s modes (areas where standing waves accumulate). Position your speakers (and subs), as much as possible, NOT in those high pressure zones. Then deal with speaker placement in terms of other considerations (imaging etc.).

It’s easier to measure from the back and I start from the wall one speaker at a time using the Sumiko Master set method, unless the speakers are designed for boundary use.

Pay attention to levelling unless measuring at the bottom.

Different wall construction also has an effect. With Naim NBLs, a boundary design, and a solid, render on brick, wall they gelled at 5.5cm. The same worked in an apartment with concrete walls but moving them to a house with dry lined walls saw the out to 33.5cm to bring them to life. This is around 78cm from the front baffle but it’s harder to be precise. I was pleasantly surprised they worked here st all and had started auditioning replacements before even trying them.