Then 225 would be too light.
The goal of clusters is to end up doing more reps per set than you normally would or use more weight for a predetermined number of reps.
In your example you have 6 reps per set. The majority of people can use 85% for 6 reps. 225 is 80% of 275 (and 78% of 285) so you are actually doing 6 reps with a weight you are capable of doing 7-8 reps with. With a cluster of 6 reps you want to use a weight you could lift 3 or 4 times in a normal set. So around 87%, or 240-245lbs.
The other approach you can use is to start with a certain weight, let's say 225.
You do mini-sets of 2 reps in your cluster and you try to get as many mini-sets as possible.
225lbs --) 2 +2+2+2 ... here you get 4 mini-sets (8 reps)
IF YOU EXCEED 6 REPS, YOU GO UP IN WEIGHT THE NEXT SET.
So you might do...
235lbs --) 2+2+2+1 .... still more than 6 total reps
The 3rd set you do
245lbs --) 2+1+1 ... oups only 4 reps ... iwhich is fine BUT I would lower the weight back down for the next set... back to 235.
So in other words if you exceed 6 add weight, if you get 4 reps or more, lower the weight. If you get 5-6 reps keep the same load.