Thanks for the quick reply Roffa.
Very interesting and much appreciated.
Sorry but still not completely clear:
If it is assumed that the BNN stack has a minimum of MSL and a maximum of FL150 then exactly what does '+' and '-' change?
If I press '-' then does it stop showing FL150; if I press it again does FL140 disappear?
If I press '+' does FL070 go away; if I press it again does FL080 disappear?
I hope you understand my questions; if not let me know and I'll try and re-phrase them in the morning