Total number of scanlines in NTSC is 262 for short frames and 263 for long frames.
So in interlace you have a total of 525 lines.
In Amiga, long frames are normally used for progressive mode.
The lines start counting from 0.
The first visible is $14 (20), because $13 is used for first sprites DMA fetch (and it is still in vertical blank).
The last 'usable' is $105 (261), because $106 (262) can only contain COLOR00 (excluding, if I'm not wrong, A1000 Agnus which can display it).
This means that for a full vertical overscan you need to use:
Code:
Copper:
dc.w $008e,$1481
dc.w $0090,$06c1
dc.w BPLCON0,$5200 ;display bitmap
dc.w $ffdf,$fffe
dc.w $0607,$fffe
dc.w COLOR00,$fff ;this show last line as white
dc.w $ffff,$fffe ;end of copper list