PROCEEDS are what was PAID TO YOU when you sold.  You then subtract BASIS to come up with a loss.  

You don't have a negative gain, you have a loss and, clearly "proceeds" can't be negative.  Enter proceeds as a positive number, enter basis as a positive number.  If basis > proceeds you'll have a loss.

Thanks.  
I understand what you are saying, but still have some confusion about how to enter this data.

Since I have more than 250 transactions, do I have to enter every single one, or can I simply aggregate all of them?  If so, how?  Is it as easy as summing some columns in my online bitcoin.tax report, or will it require more complex massaging of the data?

Also, can I simply take the bitcoin.tax total cost basis for the year and then use the bitcoin.tax 'year end value' as the turbotax 'proceeds'?

Thanks for the response!

"Since I have more than 250 transactions, do I have to enter every single one, or can I simply aggregate all of them?"

You can aggregate them and send in the detail separately.

ASSUMING the bitcoin.tax file simply lists "sells" and "basis" and resulting gain or loss it should be as simple at summing the sells and basis for long term trades and then doing the sale for short term trades.

"Year end value" suggests that it a "balance sheet" sort of number,

           i.e., (X number of coins on hand at 12/31/18) x (12/31/18 value of each coin in USD) = "year end value"

"Proceeds" is the USD received when you sold coin, (or the USD equivalent of the crypto you received.

Here's how I'd advise entering a bunch of crypto trades in the desktop version of TurboTax:

Use the "Stocks, Mutual Funds, Bonds, Other" interview and tell TurboTax you didn't get a 1099-B.  Tell TurboTax you sold "everything else".

After selecting "everything else" I'd enter a description of what was sold  like "Summary of all LT Crypto Trades - See Schedule 1" in the "Property Description" box, enter the sum of the net proceeds for the long term trades, (i.e., you owned the sold coin for more than 1 year), in the "Net Proceeds" box and "12/31/2018" in the "Date of Sale" box. 

On the next page I'd tick "Purchase" and on the "Enter Purchase Information" page I'd enter the cost basis dollars, again a summary number, and the word "various" in the "Date of Purchase" box.

TurboTax then asks if you acquired the investment before 12/31/2017 and I'd say "Yes."

And there's a summary of all your LT trades.

Go back and do the same for the ST trades, answering "No" to the "date acquired" question.

Mail in your schedule of individual trades along with Form 8453.

Tom Young