Why overcomplicate matters?
You are approaching the runway with a certain sink rate dictated by the approach angle and Your ground speed. For touchdown, You will like to break this rate to put Your wheels on the runway at an appropriate, lower sink rate.
A sloping runway, be it up or down, will influence that sink rate. An upsloping runway will come up to meet You, a downsloping one will shy away from You. In the first case, Your effective sink rate is higher, in the second case, lower. So with an upsloping runway, flare a tad earlier and more; with a downsloping one, later and a little less. If done right, You will achieve roughly the same body angle on touchdown relative to the runway, whatever the slope may be.
Looking down the runway... this may help in some cases, in others, it will not. Not all runways show the same slope all along their length; some are a little wavy (Birmingham comes to mind), some may be U-shaped, others are mostly level but have a little upslope right in the touchdown area, they are all different. Consider judging the sink rate based on the runway edge, keeping in mind that different width runways look different - this seems to work for me quite nicely most of the time (the occasional solid arrival does happen) and also helps in case of variable slope runways.