Development help with HP5 8x10 under exposed

[iamodell]

Not paying attention, I misapplied my bellows extension factor.
Instead of increasing my exposure 2/3 of a stop, I decreased 2/3 of a stop,
so I effectively under exposed by a stop and a third.

Typically my development time for HP5 is 13 minutes.
I was shooting in the studio, tungsten lights, portraits, with an EI of 320.

Can anyone suggest how much additional developing time I should use ?
I have not worked out my n+ or n- times as yet.

Thanks,
Charles

[koraks]

As a rule, something like +25-30% should bring the contrast back on track, but there's no recovery of lost shadows of course.

[pau3]

What do you plan to do with the negative? If you’ll use an enlarger, 25% or 30% will probably enough. If you’re contact printing with weird chemistry (platinum or whatever) you’d probably want to give a little more time, close to 50%. Shadows will depend on the constrast of the scene.

[Thalmees]

Compensating development is another option.
Also, you may post-expose and place it at zone II, plus extending development not more than N+1.
When you could not find diffused out of focus area except by extending your bellows, do not forget to compensate.
Good Luck.

[Bruce Watson]

As a rule, something like +25-30% should bring the contrast back on track, but there's no recovery of lost shadows of course.

Yup. This. What you have is a 1 1/3 stop "push". But pushing is really just under exposure that you try to "fix" with over development. But as Koraks says, you can't recover the deep shadow detail because you didn't record it -- so there's nothing to recover.

Anyway, I'd go with 25-30% increase in your normal development time and hope for the best.