In a former lifetime, I worked "under the thumb" of the General Accounting Office (GAO), and was required to submit detailed "Cost Estimate Worksheets" and updates in support of engineering programs for the US Air Force, Navy, and to lesser extent the Army.
Each contractor is required to "establish from experience records" a "Basis" for their estimates. This is done by keeping fairly detailed records of time required to accomplish specific "tasks" on each program - almost as much for use in estimating the next task as for actual billing on the one being recorded.
We were permitted not too long ago to "estimate" 8 hours per "A-Size" (8.5 x 11 inch) page. (It used to be "MH" for manhours, but I suppose by now it's PH for "personhours.) Since a typical page is about 20 to 24 lines, this fits well with your 20 lines per day.
Accuracy of the estimates could be improved by an allowance for "complex pages," at 16 hr per page. For a "typical component design specification" of about 120 pages, up to 30 percent might be "complex."
It should be noted that the 8 hr per page is the time required by the person who "prepares" the document. A "Burden Rate" is applied to this estimate that factors in something like - for example - 3 hours for a drafter, 1.5 hours for a secretary, 0.6 hours for "managers", etc, for each 8 hours required by the preparer. The "technical preparer's" $25 per hour "wage" might well be bid at something around $125 or higher per hour in "total burden rate."
Properly used, these "factors" permitted very accurate task analysis - if, but only if, they are conscientiously and frequently audited and maintained during development of a program.