ur approximation is nearly fine ,,,, usually it take 8-8.5 hours including some stays and brunch at ayubia,,, it was something like that
1 hours from Nathiagali-Mukshpuri
20 minutes from Mukshpuri-Dunga Gali (its just decending,,,, u can do it within 10 min as well upto metalled road) 
30 minutes from Dunga Gali-Ayubia
2.5 hours from Ayubia-Khaira gali
2 hours from Khaira Gali-Murree (stay only for rest,,, no particular sight seeing,,,, in my opinion is more tiring,,,, so if one could get van,,, it would be better)