A couple of observations - already partially covered by chaps1954. According to the ACL Start of Season report for S19 Ethiopian have 135 seats per flight from Manchester. This equates to approximately 30% of the aircraft capacity and so if this is correct then the Manchester loads are within an acceptable level.
Singapore Airlines is a harder one to ascertain as the aircraft operates as a through service from Singapore to Houston. Again according to the ACL report the number of seats available from Manchester to Houston is 194 against the aircraft capacity of 253. On this basis Manchester should be looking for a load-factor of approximately 74% of the full aircraft configuration to be operating at 100%.
In both cases I have used the total aircraft capacity for my calculations.
The load-factor calculations of mine are still in their infancy as I am finding, for an example that both Air Transat and Thomas Cook don't stick to a single flight number on a route, but have multiple numbers. I'm doing a daily search of the FR24 database to find anything that I've missed - in the same way that Emirates have various configurations on their A380 and therefore, to be truly accurate, I need to check exactly which aircraft is being used in each flight.
I hope that helps a little.