For this question I'll stay away from parametric equalisers that allow you to alter f and Q, which can be very useful for electric guitars, as you may want these in different places when you change the effects/distortion used, and stick to the "classic" graphic equaliser used to just use a high pass filter at the lowest frequency you want to control, a low pass filter at your top frequency, and a range of bandpass filters usually logarithmically spaced between those two extremes. And this works reasonably well for a large range of use cases.
There are some straightforward equations that give you your 3dB bandwidth frequencies, and Q factor, and this page is an excellent resource, but before you get to that you need to be aware there is no right answer to how much overlap each band should have, so it is worth looking at examples.
From your building blocks, you will be able to alter your frequency and Q factor. The 3dB bandwidth from a particular Q factor is outlined in this table from the page I linked to:
But that won't tell you whether you should choose 3 octaves or 1 octave. That is very much down to what you prefer. Looking further down that page, you will see a Yamaha Parametric EQ example. I tend to err on the side of slightly more overlap to avoid unwanted notches if I use this type of EQ, but I prefer the greater control and tweakability of a full parametric EQ.
The sengpielaudio.com site actually has all the equations you would need to work this out yourself, and most importantly, has one of the two simple cutoff frequency calculators I have used when needed. If you can I would build a set and experiment with a range of inputs, both single instruments, and entire orchestras and everything in between to see what works in different situations.