I did exactly this hard slog in Y a week ago. Luckily my QP membership allows me to pre-allocate seats in the mini-cabin (Rows 26, 27 and 28) before J begins. Row 26 is especially good, as you have generous legroom, are usually the first to be served meals and drinks and often among the first I was second

) to disembark depending on which door is used. In Singapore it's usually aft of this mini-cabin, in Frankfurt mostly the very front left-hand door. The downside is the curtain can be annoying, especially when stressed flight attendants are pounding up and down between cabins. Also, the seats in this row are slightly less wide than others due to the meal tray and monitor being stowed in the armrests which, consequently, are also fixed in place. I have attached a link to a video I made whilst we were waiting for thunderstorms to clear in Sydney. The beautiful feet you will see belong to me. FYI I am 178cm.
http://au.youtube.com/watch?v=6a4vBhQx1Oo#
I can recommend: 26ACHK. I would forget about the middle seat even though it too has good legroom. Also worth looking at are 29BC, but unless you are a WP you won't be able to pre allocate these. I believe some travellers also enjoy 35HJK as they are located directly behind the crew rest seats (the benefit being that they are rarely reclined). However, I found that they can be a little claustrophobia-inducing as they are completely enclosed by curtains.
If you have a break in Singapore you may wish to try allocating seats for the SYD-SIN sector and then different seats for the SIN-FRA leg. Whatever you decide to do, enjoy the flight (and being woken up at 3am FRA time for a rubbery omelette and some soggy bacon

).