It depends on your pH meter, lots of the cheap ones are only single point calibration capable, if you do a second point it promptly forgets the first one. That being the case just calibrate with the closest buffer you have.
Better quality meters fix the first point, then adjust the slope of the line between the first and second point. If you want to be technical it’s an application of y=mx+b, the first point sets b and the second adjusts m (gradient).
pH plays a huge role in brewing, not just in mashing and the kettle, perhaps a lot more important than most home brewers realise. Many commercial brewers monitor it as closely as they do SG even during fermentation, it gives you a lot of information on yeast health and even plays a major role on when to dry hop or rack beer.
Most blond beers should finish around 4.1pH give or take a point or two; it can have a big impact on the finished beer. pH also tells a brewer a lot about how well your processes ar working.
If you have a decent pH meter calibrate it at 4pH and do some trials, take three glasses of beer, adjust one up a couple of points, the other down the same and see if it affects the beer, in some beers it makes a startling difference, others not so much. I use Potassium Bicarbonate and Lactic acid respectively and it doesn’t take much so best to make a solution/dilution and add a little, stir gently, remember to stir the blank sample the same amount as the dissolved CO2 will change the taste to.
Mark