>Why the arbitrary length of 20? The article suggests the workaround of changing it to a string and back to numeric, so how about just

ROUND( VAL( TRANSFORM(tnNumber)), tnDecimals)

?transform(tnnumber)
tnnumber=2.0000000056984984924892482948294829
?transform(tnnumber)
tnnumber=2.00000000569
tnnumber=0.00000000569
?transform(tnnumber)
tnnumber=0.02000000569
?transform(tnnumber)
?val(transform(tnnumber))

The maximum number of decimals, BTW, is 12, as I just found.