In my project initially R2 was 4k7, but now I modified it to 3k9 as it was in @xavier60 original schematic. With 4k7 the max output voltage was about 27V, now with 3k9 it is 32.77V.
Please find attached the screenshots:
1. CH1 on PSU output, CH2 on 555 output, with Rload = 0 R - 0038, 0039, 0040
2. CH1 on R_Shunt, CH2 on 555 output, Rload = 0 R - 0041, 0042
3. CH1 on PSU output, CH2 on 555 output, Rload = 10R - 0043, 0044, 0045.png
I used a 32Vac/200VA step down power transformer.