Financial HP Calculator: Amortisation with Payments at begin of period

[Gil]

"Do you mean the gradually growing payments? No, I leave them at full precision and use the rounded value for interest and balance calculations, but I do not store the rounded value in the PMT variable."

If interests are calculated to the cent, ie with 2 digits after the comma, then if PMT1 is found equal to 2511.73389, the client should start paying — theorically—, in the armotization plan, exactly (2511.73389; 2 RND) = 2511.73.
It seems, however, that it is not what I understood from the above.

From my logic above, but I seem to be wrong?, if the growing payments show a yearly increase of 5%, then the second payment should be equal to ("exactly 2511.73" ×( 1+5/100/12); 2 RND). Or not?

Another point
Why do you use negative integer before RND command?
Is full precision not equivalent to simple 12 RND?
So that x RND, with x=0, 1, 2...11,12, should be enough to tackle all possible choices. Right?

Thanks again for your explanations and co-operation, Werner.

Regards,
Gil