GOLD 1 = between 1249 through 1495 DIAMOND 4 = between 1496 through 1585 DIAMOND 3 = between 1586 through 1651
1488 means you are standing on the chandelier, almost touching the high ceiling on Gold 1 (Diamond 4 is the floor above). Below, you can see someone with 1250 walking around on the shiny golden floor.
Yes, because (all reward) tiers are percentage based, they will slowly creep up (mostly) or down depending on the level of scores and number of participants.
Because of that you have to adjust what your 'target tier' goal score will be and will have to 'pad' it with some 'extra' points in case it creeps up.