Making a PARTIAL Private Residence Relief claim on a loss?

Can anyone help me please on how to claim partial - not full - Private Residence Relief on my Self Assessment Return? The home I sold, at a net loss, was my main / only residence for part of the time I owned it, and it was rented out or acted as my second home for the rest of the time. I've done the date calculations and worked out that the period it was my main / only home, plus the final 9 months of ownership, represent 31% of the total period of ownership. So I believe I can offset only 69% of the total net loss against other capital gains (on shares/funds disposals). But how do I present this partial PRR loss? On the "Capital Gains Tax summary" form SA108, I can insert the relevant "PRR" code in Box 8, and I assume I include the FULL (100%) amounts of the disposal proceeds, allowable costs, gains, losses, in Boxes 4 to 7, not just pro-rata 69% of each. If that's right, how and where do I reflect the fact that I'm only claiming to offset 69% of the net loss, not 100%? I can't see anything in the SA108 Notes to explain this. I had formally nominated to HMRC when the property became and later ceased to be my PPR. Thank you.

Hi

PRR is calculated as the gain (after deductions) multiplied by the period of main residence over the period of ownership, eg  £100000 * (40+9)/110 = £44546.  £100000 - £44546 = £55454 that is charged to tax.  
If the PRR covers all of the gain, then there is no tax to pay.  The tax return should show the disposal value, the allowble costs should inclue the calculated PRR, leaving a gain that SA will apply the annual exempt allowance to.  
Have a look at helpsheet HS286 at HS283 Private Residence Relief (2023).

Thank you