So there's where I am now:

Desired freq = 38kHz (PWMperiod = 26,3157µs), crystal (Fosc) 20MHz, prescaler = 1
SEARCHING FOR PTPER (max 4095)

Transform formula:
PWMperiod[µs] = (PTPER+1 * prescaler) / (Fosc / 4)
=> (PTER+1 * prescaler) = PWMperiod * (Fosc / 4)
=> PTPER+1 = (PWMperiod * (Fosc / 4)) / prescaler
(26,3157µs * 5'000'000) / 1 = 131

But how do I calculate the duty-cyle register (PDC0) admitting I'd like to have a 50% value to set?