You need one centimeter of extra space minimum.
You can gauge this quite simply by unlacing the boots well. Crawl the toes to the end till they touch comfortably. Foot flat on the ground and standing. Tilt knee forward so you can get your finger behind and feel for that space.
Now lace up boots firmly and kick a carpeted step firmly. You should not get hard impact for toes. A little is the most and indicates the lacing retains the foot ok.
No tightness for toes. Some between the toe balls to heel is ok in thinner leather boots as these can stretch more but not very much for 2.6mm thick quality leather or more. Where the rand is which is the rubber that covers the leather for abrasion protection and support, there will be next to zero adjustment for molding to the foot. If you have tightness you need to consider how much that is and if its going to be too tight to breakin to form to your footshape. Its reasonable that some pressure points are felt on initial fitting then are no longer noticed after walking in them for a while. The beauty of leather is it moulds to the wear. But this can take time which is why its called breaking in the boot. You can soften pressure spots faster by applying some oil which will soften the leather.
Thinner leather as said more easily adjusts to the shape.
Tell us which brand and model you have worn before as we can compare those from passed orders. Also do our measurement and let us know you toe profile as different feet shapes fit different boot shapes. We have a good range and can fit nearly everyone. We don’t have over UK 13.