For any Business letterthis area is tricky because you may or may not know your audience.
If you do know your audience, you can tailor your letter to that audience. However, most of time your audience is larger than you expect.
If your letter is addressed to some high profile (say- XYZ ), remember and assume that your letter will probably be read by several other people working for or in XYZ's office to receive the action or convey the message you wish to.
If you are unsure of your audience, assume they are educated, reasonable people until you find out otherwise.
Don’t assume they have as much knowledge of the subject of your letter as you do, or you may overgeneralize or forget to include important details.