And for example, $2000 in electicity expenses - i enter the total $2000 under business-use-of-home and it calculates. Do i enter $2000 in electricity under farming expenses as well?
If your "office expenses" are incurred only for your Farming Business, you only enter these once as an "expense" on the T2042 Statement of Farming Activities. (They are not entered again under the Business Use of Home, so in your case only enter the $1,000 in the farming expenses section).
As for your electricity, CRA says the "Only the part of your electricity costs that relates to your farming business is deductible". To determine the part you can deduct, keep a separate record of the amounts that apply to the farmhouse, and to other farm properties. (For example, the business part of your electricity expense will depend on how much electricity you used for the barns and shops. So if you used 50% of that $2,000 for actual "farming expenses", enter $1,000 in the farming expense section, and the remainder in the Business Use of Home section).