Jwscud
Below 50 ft. you go by what you see and feel. With a slope towards you, the rate of descent induced by flare is in effect reduced by rate at which runway is rising.you get a feeling that you have not flared enough as the runway rushes towards you so subsequent change of attitude needs to be sooner than normal runway. Opposite of this happens when it slopes away from you. You feel you have ballooned and there is a bit of unflare required. Ultimately you need to follow the slope during flare and landing. If you do not cater for this you are likely to get harder or firmer landings upslope and landing deep with downslope. IIRC Abu dhabi runway is an example of upslope. Bombay runway 14 has a downslope after some distance from threshold. If you do not touch down early it gives a feeling of ballooning.