Can you double check the multi$-$specimen method solution? I put the numbers in the problem into that formula and I$\text{'}$m getting that the change in J from $24->25$ is nearly double J$24.$ If thats correct then so be it$,$ but it feels like this should be a relatively small change for $1$ mm of crack length difference.

For reference, I used the J$=$ U$/($B$*$a$)$ to find that J$24=450$ N$/$mm$.$ I then used the J$=$delta$\_$U$/($B$*$delta$\_$a$)$ with delta U for $24-25$ being $8$ kN$*$mm $($U$24$ should be $54$kN$*2$mm$,$ and U$25$ is $50$kN$*2$mm$),$ B $=10$ mm$,$ and delta$\_$a $=1$mm$.$ This gives me a delta$\_$J of $800$ N$/$mm$,$ which is nearly double J$24$