Depth from manhole cover to invert are:- 4980 and 6280 . Difference on level of manhole covers 3000 minus 3070
Difference in level of inverts 6280 minus 4980 equals 1300. Less difference in cover level 070 equals 1230
Size of pipe 900mm, material GRP. Distance between manholes 156520
Slope off pipe :- 156520/1230 equals 1: 127 approx.
Slope of 1/127 is normal for pipe of this size. Dimensions in mm for clarity.
Formula for slope is the distance between two points, measured horizontally, divided by difference in level, measured vertically. Always using same units of measurement. I suspect you are seeking a formula for something different. What is the nature of your intended works.
For estimating purpose- 4.980 is so close to 5 metres I would ignore the difference. This would give you :-
Length of pipe between 5 and 6 metres deep, equals 127 metre. Pro rata if B of Q has different splits.
Length of pipe between 6 and 6.3 metres deep (156.52 minus 127) equals 30 metres.
More accurate calculation :-
Difference on depth between both points 1.300 over distance of 156.52 metres. Slope of pipe relative to slope of ground 120:1 if B of Q splits depth at 300mm each 300mm will contain 36 metres of pipe.