Compatible RAM to pair for Laptop?

[jasonsmith55559]

How precise does the speed and timings have to be when adding a new stick?
Ive read a couple of places that matching 2 different pair of sticks can be a coin flip if it works.

RAM specs
http://i.imgur.com/BaBsEbn.png
http://i.imgur.com/nkBPkIi.png

Looks to be DDR4-2400 (1200 MHz) with 15-15-15-35 timings at 1066 or 17-17-17-39 at 1200.

This one https://www.newegg.ca/Product/Product.aspx?Item=N82E16820232153
same speed but timings slighty off at 16-16-16-39. Is that a big deal?

This one https://www.newegg.ca/Product/Product.aspx?Item=0AY-00PT-00008
http://www.crucial.com/usa/en/ct8g4sfd824a
same speed, all it tells me for timing is CL=17 and dual ranked

this similiar crucial one https://www.amazon.ca/dp/B01BIWKP58/ref=asc_df_B01BIWKP581502794800000/?tag=shopbotca-20&creative=395285&creativeASIN=B01BIWKP58&linkCode=df0
http://www.crucial.com/usa/en/ct8g4sfs824a
same speed, same CL=17 but single ranked

There is an ebay option for the exact same model provided with the laptop.
http://www.ebay.com/itm/8GB-DDR4-1Rx8-PC4-2400T-2400MHz-SK-Hynix-RAM-Laptop-SODIMM-HMA81GS6AFR8N-UH-/162623327636?hash=item25dd1b0d94:g:9EMAAOSwbopZUskK
but it looks to be used listings.

What is the best option to make this upgrade as smooth as possible?

 

[The Paladin]

just buy Identical stick you already have, look for model on the stick search for it
you mention an ebay option of exact model : HMA81GS6AFR8N-UH
if that is the model then you have these choices

http://www.memory4less.com/hynix-8gb-sodimm-pc19200-hma81gs6afr8n-uh
http://www.discountmicrosales.com/hma81gs6afr8n-uh.html
https://www.ubbcentral.com/store/item/NEW-SK-HYNIX-8GB-SO-DIMM-DDR4-12V-2400-MEMORY-HMA81GS6AFR8N-UH_192118962857.html
https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=HMA81GS6AFR8N-UH+&rh=i%3Aaps%2Ck%3AHMA81GS6AFR8N-UH+