This is a bug bear of mine. I keep hearing 'over pitching is just as bad as under pitching' which I disagree with because if you over pitch you get the beer just not what you might have wanted, if you under pitch you might not actually get beer at all.
Also who defines over and under pitching? The preference of the brewer comes into this. Maybe you are looking to under pitch to emphasise ester production during a longer lag phase? Maybe you are looking to over pitch because you want to do the opposite or you need the beer to finish particularly quickly? Maybe you are working at higher or lower temperature ranges and need to manipulate the pitch rate to prevent the yeast coming over? Maybe you propose a secondary fermentation, a lager, an extended cold crash, a long conditioning process? Even so you aren't going to step too wildly out of a standard margin unless you have the luxury of doing something truly weird.
The reason why professional advice looks very heavy on the yeast is because primary fermentation is complete in 2-5 days for a standard ale under normal conditions. Home brewers have a relative luxury of time compared to commercial brewers and thus you can do some weirder things though you've got to start somewhere. Pitching rates as viable cells/ml need to scale with gravity. Pitch 0.75 to 1 million cells per ml of wort multiplied by degrees plato as a guideline unless you have other plans for a standard ale under standard conditions.
Eyeball this by reading the manufacturers spec sheets for your yeast and scale accordingly. Don't forget to apply a fudge factor for viability. How can you improve on this? If you really get into maintaining and reusing your own strains get a microscope, haemocytometer and start performing cell counts. If you are using slurry and you do not have access to this equipment you'll have to estimate based on weight which luckily is quite often good enough. Why do people make starters? Because it ensures you've got 'about enough based on experience'.
Under pitch and you risk a long lag phase with a risk of contamination as well as unexpected flavours. You might not have enough yeast left to finish the fermentation in bigger beers or complete bottle or cask conditioning if appropriate and it will be difficult to crop. I'd also anticipate greater genetic drift over time due to greater number of divisions. Over pitch and you might get unexpected flavours, an aggressive fermentation requiring extra room in the fermenter, too high a cell count in the packaged beer with associated clarity and autolysis issues and longer term viability issues when cropping.
If you can't accurately measure everything the key is to start somewhere and remain consistent. If you are unhappy with the results you can start to adjust one variable at a time until you reach your goals if you remain consistent with your practices you'll get to the same place in the end.
I top crop on the second day of fermentation and chill the yeast. I decant off any liquid and consolidate the solid yeast. The next day I decant off any liquid and weigh out. At the point of cropping it is approx 4.4 x 10^7 cells per ml at 92-94% viability. It typically loses 5% viability per day in the fridge. We pitch 3-4 days a week so it doesn't hang around long.
Ideal pitch rates allow 2-3 division of cells before the yeast fully exhausts oxygen and trace elements within the wort during the reproductive phase. It then must go anaerobic and start primary fermentation. You need this division not only for certain flavours to be produced, but for the yeast to have a range of age across generations in order for enough to stay alive long enough to complete the fermentation and any secondary fermentation and conditioning.
If you pitch all the yeast required for the fermentation at the beginning then it'll quickly consume the oxygen and trace elements and you'll be beginning the fermentation with yeast already close to the end of its life span. Any daughter cells produced will not have access to nutrients required to prepare for fermentation. You risk an incomplete fermentation, a failure to condition and potential issues with flocculation as dead cells do not clump together.
That said it'll still be beer.
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