The 5mg daily low dose Tadalafil gives you equivalent to an 8.5mg once-off dose, because of the compounding and half-life from previous days too. That's quite some way off the 20mg event dose. Although thought to be good for penile rehab for blood flow, the daily low dose is insufficient for those needing substantial PDE5i help to get erections, but many people needing PDE5i help don't actually need the full 20mg, and the 5mg daily low dose is enough for them for regular erections.
If you're on 5mg daily low dose and you want the effect of an event dose, you need to take 3 of the 5mg tablets in one day, and can then skip them the following 2 days to get back in sync with your calendar pack. That gives you the equivalent of 18.5mg once-off that day, very close to the 20mg event dose. The day afterwards you are still at same dosing level as if you'd just had 5mg daily, and the following 2 days (which includes the day you restart taking 5mg daily) you are below the 5mg daily dosing level, but still have some continuous dosing.
Both the 5mg and 20mg tablets are available half size, 2.5mg and 10mg. These are for people who find they get unacceptable side effects, but the half dose is OK regarding both intended effect and side effect. The half dose tablets aren't usually cheaper though.