I urge you to find a bandsaw for this if you can , however if you can't I assume you can get your mitts on a 'Snotter. If the board is 60 mm then you need to cut from each edge about 35mm ( thats the depth of the blade in use ) the problem you have is keeping the saw stable and at 90 deg to the edge of the blank. Think in terms of clamping a couple of bits of 2 x 2 to either side of it to keep the base of the saw flat , but then you need to find a way of guiding the saw down the middle of the blank. You said 20mm so takeaway 3mm for the saw cut leaves lets say 16mm at best therefore gives 8mm per. Thats plenty for a fretboard assuming you dont need to plane off too much. The bandsaw will take less on the cut so you will be golden if you can get it done that way.
Good hacking chief.