Unless you have an 80 gallon boiler at home, you’ll have to scale the recipes to fit the equipment you have. An awareness of the units the author is using is useful to keep the recipe reasonably accurate.
Harrison (1577) uses the bushel as his primary measure of dry volume for malt and grain. The most likely is the 1558 London grain bushel, of 35.27L, Digby (1644) uses an ale hogshead (221.82L) for wet measures and peck (9.0921L) for dry (Unless measuring Barley, oats or malt, in which case, 1 peck = 13 L) and Markham (1615) uses a “quarter” of eight southern, or four northern bushels (standardised by the Parliament of Scotland in 1661 and abolished by the Parliament of England in 1824 – roughly 280L). The standard ale gallon was set at a volume equivalent to 4.62L in 1454 and wasn’t changed again until 1688. The ale hogshead was 48 ale gallons (221.82L) but the beer hogshead was 54 ale gallons (249.54L). Of course, one or more of them may have been using the Queen Anne gallon (6.831L). Be aware that modern US volumes (US gallon = 3.78l) are quite different from the equivalent Imperial measure (UK gallon = 4.54l) and can create an error of up to 20% if you use the wrong one.
Weights can be problematic as well. Harrison uses Troy ounces (14t oz. = 1t lb = 3.73kg), most of the others use avoirdupois ounce (16oz. = 1lb). Fortunately, the bloke at the local brew shop was able to tell us 1L of medium ground (33 micron) barley weighs near enough to 600g.
Recipes on the Internet often make it worse as the author uses their local units while not knowing/understanding/caring what units the original was referring to. UK Imperial (453.592g), the Troy (373.241g, abolished Jan 6, 1879), the Apothecaries (also 373.241g, abolished 1 Jan 1971) and US Customary (453.597g) pounds all vary. Mercifully, the Tower Pound (14 oz T = 1 lb T = 349.91g) has been out of use since 1528. The London pound (16 oz T = 1 lb L = 466.66g) survived a little longer.
You’ll find most pre 1600 recipes use Troy ponds and most post that date use the UK Imperial, while many people reproducing these for Internet publication assume all measures are US customary weights and volumes. The problem is that the proportions of Imperial to US weights are different to the proportions of Imperial to US volumes leading to completely different results from those originally intended. I find it better to convert all the recipes into modern, metric units before scaling it to fit my (ahem!) equipment. For Mrs Harrison’s Ale, I came out with 1.25% of the original quantities. Conversely, with Digby’s Small Ale for the Stone, it was 62.5%, as this was made in small batches for quick drinking while fresh.
As long as you have an idea of what you are doing, you should be able to get by working on the proportions of what they used to what you need. For example, Harrison used a ratio of 16:1:1 for his barley, wheat and oats. With his yield of 0.9 (360 gallons of ale from a start of 400 gallons) we can easily scale to modern fermenters that are typically 21-23 L. The only complication is then in calculating the quantity hops and the amount of yeast. Typically, you’ll be using less than 30g of hops as they were lightly used due to expense prior to about 1720, and a packet of dry or 100ml of wet yeast.