How to set marks to measure against?

[gumbo23]

I am creating a book cover, and trying to line up elements against the template in the background, and get some thing central within the margins etc.

Is there a way to set ad hoc marks, and then have selected elements measured against them? Ideally I would be able to also center them with one click.

I've added a little screenshot for illustration.

Thanks

[Alfred]

It sounds as though you need ruler guides and/or column guides.

[gumbo23]

2 minutes ago, Alfred said:

It sounds as though you need ruler guides and/or column guides.

I should have marked this as being in Photo. 

I've tried dragging out rulers but they vanish. Now sure how to get them to stick and help me out in the way I require.

[Old Bruce]

1 minute ago, gumbo23 said:

I've tried dragging out rulers but they vanish. Now sure how to get them to stick and help me out in the way I require.

Do you have View > Show Guides checked?

[gumbo23]

Just now, Old Bruce said:

Do you have View > Show Guides checked?

Yes

[Alfred]

2 minutes ago, gumbo23 said:

I should have marked this as being in Photo. 

Ruler and column guides are available in Designer and Photo, too.

Affinity Photo Help: Ruler and column guides

If, as you say, you have ‘View > Show Guides’ checked, you should be able to see the guides as long as they’re over the canvas.

[gumbo23]

1 minute ago, Alfred said:

Ruler and column guides are available in Designer and Photo, too.

Affinity Photo Help: Ruler and column guides

If, as you say, you have ‘View > Show Guides’ checked, you should be able to see the guides as long as they’re over the canvas.

How would they not be over the canvas? They are top when I drag them out, but then they disappear when I release the mouse.

[GarryP]

I don't want to confuse the discussion about Guides but does what gumbo23 wants to do actually need them?
I might not have understood the requirements properly but it sounds like you can draw a (temporary) Rectangle across the required area and use Snapping to get the centre points of that Rectangle. (The rectangle can be made invisible when it's not needed, or deleted if it will never be used again.) Or have I got the wrong end of the stick, requirements-wise?

[gumbo23]

2 minutes ago, GarryP said:

I don't want to confuse the discussion about Guides but does what gumbo23 wants to do actually need them?
I might not have understood the requirements properly but it sounds like you can draw a (temporary) Rectangle across the required area and use Snapping to get the centre points of that Rectangle. (The rectangle can be made invisible when it's not needed, or deleted if it will never be used again.) Or have I got the wrong end of the stick, requirements-wise?

This should work fine Garry - I'll try it. This is why I ask you guys!

[GarryP]

It would be interesting to hear back from you as to whether this was a workable solution for you – it might help others, or maybe it won’t.

I would also be interested to know if there was a solution to your guides ‘disappearing’ as that doesn’t sound like it should be happening.