You're in section B so you get a lump sum as standard, which is typically worked out as 3 times your pension. You now also have the DBCBS from April 2018.
If your standard lump sum is lower than 25%, then you can use the DBCBS money to top it up to 25%, and then anything left over would be taxable. According to the info we received earlier in the year, you could also transfer it out for drawdown or annuity purchase. Details
here.
But as you're in section B, you also have the choice(assuming nothing has changed that I haven't heard about, as it's not mentioned in the latest plan guide) to commute some of your standard lump sum into more index linked pension. Meaning you have a bigger pension and you can use more or all of your DBCBS cash to fund the tax free lump sum.
So there's potentially no need to reduce your pension to get the maximum lump sums from your NRA60 & NRA65 pensions, as you seem to be suggesting? Depending on exact figures, you could actually get a bigger pension and the maximum lump sum!
From posts on these forums, the commutation factor for section C members to give up some pension for a lump sum, seems to be around 1:22, meaning for each £1 they give up in yearly pension, they get £22 in tax free cash.
To be honest I don't know if the commutation rate for section B members to do the opposite, and it may well change depending on what the schemes actuaries decide. But if you use the same formula, but the other way round, it should be in the right ballpark.
I'm sure the pensions dept. will give you the details about your options at the time.