On the 146 the weight is kinda seperate from the Flex calculation.
What does change is your take off distance and V1 as you'll likely be taking longer to roll/climb etc etc. Until someone makes a publically available take off calculator just gonna have to use the brain a bit.
There is no chart or limitation for payload vs. a flex power setting.
In practice - at the operator I'm familiar with - taking off with a light load or a very long runway in the winter, is the only time flex power was used in the 146. Most of the time come spring/summer the engine was already limited to a reduced N1 as you see in the above charts. So any kind of wear and tear benefit was miniscule.
On the Avro the TMP makes things little easier, and the better performance calculating and engine condition monitoring tools (Engine Life Computer) make the REDU thrust takeoffs used a lot more.