Upper and lower bound - Digital electronics
Here, your calculation seems to be correct,

VIL - VOL = NML
VOH - VIH = NMH, from your calculation, NML = 2.3 V - 1.4 V = 0.9 V and 
1.8 V (NMH) = VOH - 4.5 V, so VOH = 6.3 

From your valuation we can say that, If Vdd is 5 V then VOH can't be 6.3. See Image and interpret accordingly.