You cannot be absolute in these matters. The business letterhead plus the placing of the window in the envelope plus the length of the letter itself will govern positioning of various parts of business letters.
However, a norm can be stated for one or two line spaces between the last line of the recipient's address and the salutation, one line space to the start of the letter; one line space between the end of the letter and the complimentary ending, 4 or five line spaces for the signature.
Work out what you like the letter to look like, and then use that as your template.
See here: Formal Letter Writing - Articles - UsingEnglish.com
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