How can I get my total gain to be worked out from the bid price (not the last price)

Portfolio holdings are valued by default at the last traded price, though you can edit this within your portfolio settings to calculate valuation at the bid price if preferred.

To do this, click the 'Settings' button at the top of you Folio page to open the Folio Settings modal.

In the Folio Settings modal you can change a number of details about the Folio set-up, including the valuation method.

You can adjust the folio valuation method by selecting either Last Price of Bid Price. Click Save to save your settings.

You can read more about Folio valuations here.