how to predict the Windows.old directory when doing In-Place Upgrade

When servicing customer for Windows 10 In-place Upgrade, for example form 1607 to 1803 using In-place upgrade, how to predict the Windows.old folder size will be created.

1. how to improve the update speed, can I bypass the auto backup sequence? 

2. If 200GB disk size already used in 256GB disk, is it enough space for upgrade?