calc liter per hour by MAP - how?

[MGTF135]

I´m not sure if I´m right here but I´ll try in my bad english.

my car (MG TF) has no MAF-Sensor so I couldn´t use the formula:

GPH = MAF * 0.0805

but I read, that MAP (mainfold air preasure) could be used to calc MAF

MAF = RPM * MAP / absolute temperatur (PID15)

if I do this MAF = 960 * 37 kPa / 27 °C = 1315

I know a engine like my 1,8L 135PS need 2 oder 3 Liter, so GPH = 0.6
with this should MAF be 7, if I calc with my OBD-value I got 1315, hopefully everyone unterstood the problem ...

Do someone know a way, to calc with speed, rpm, mainfold air preasure and temperature GPH or Liter per hour ... the change form gallone to liter is not my problem???