One Space or Two?

Q. Please help. I have confusion regarding the correct spacing after periods and other closing punctuation. My company uses the font Arial and consistently uses a flush-left margin. We are an engineering company. My job consists in preparing documents and letters for customers. Everything I read in manuals and from technical writers directs me to use one space after periods. I find that it works very well, except occasionally, when an extra space helps readability. Knowledgeable engineers have embraced the one space use as being consistent with the font design and automation of reports. Others are unpleased with the one space, they think they have difficulty reading. (I, too, had an adjustment period which I forced myself to endure until it became automatic to read easily.) We are preparing technical information. What do you think? Thanks for your wonderful support and especially the quick answers. I greatly appreciate your service.

Q. Help. How many spaces do you leave after a colon in a manuscript?

Q. About two spaces after a period. As a US Marine, I know that what’s right is right and you are wrong. I declare it once and for all aesthetically more appealing to have two spaces after a period. If you refuse to alter your bullheadedness, I will petition the commandant to allow me to take one Marine detail to conquer your organization and impose my rule. Thou shalt place two spaces after a period. Period. Semper Fidelis.

Q. All right then! I’m steadfastly attempting to adopt the “one space after concluding punctuation” rule. It’s not an easy task for a retired English teacher in his late sixties—one who preached the old rule to legions of eager-eyed scholars. Are there any retraining suggestions that assist the elder learner? I’m tired of correcting my continual errors. I am diligently trying, though.