Personally, I think there is an element of excitement in the first times using Sildenafil. It's something new - kind of a sex toy - and, for me anyway, I think that guaranteed rock hard boners the first few times, irrespective of the dosage I took.
For me I find less than 50 mg is good for avoiding headache or general wooziness (though I also take ibuprofen and paracetamol). I'm not great at cutting a 100 mg into 4 pieces so I usually cut it in two, and then cut a bit more off one of those two pieces, and then take what remains. Still get the red face and a reliable boner (so long as I'm in the mood for a punt).