SG210, SG230 or SG310? User calculation question

Hello, 

We are looking for a new firewall. But if I use the calculation, I don't know if I'm do it right. So my question is, do I make the right calculation?

Internet-speed: 500 down, 50 up. 5 externe ip addresses.
Total networks: 5

Network 1: Office
25 Workstations, 7 heavy users (more than 250 mails a day(Office 365)).
25 Phones form employers
25 VoIP phones, max 8 at the same time calling outside.
5 VPN users (home working)
1 VPN tunnel (10~15 users, + MySQL sync every 5 min. + once a day backup(data, 25GB)). They have the SG210. 
Windows FileMaker server (authentication with AD) for all users (25 workstations + VPN tunnel users)
Synology NAS (Time Machine only, 25 workstations) connect with the AD.
Synology NAS for file sharing (25 workstations) connect with the AD.
Synology NAS for backup, this NAS will sync the backups every night too an other Synology NAS to the VPN tunnel

Network 2: Repairs/Testing
±10~15 Mac connected with internet (mostly youtube/webbrowsing/some mailbox updating)
±50~75 iPhones connected with internet (mostly youtube/iTunes)

Installing updates for all devices (4 central points where the updates are downloaded), so this is including into the devices (iPhones/Mac’s) up here.

Network 3: Servers
3x webservers with Plesk (total 30.000 visitors a year)
10x Synology NAS for backup from costumers


Network 4: Costumers network
max. 15~20 costumers connected at the same time (internet, mail). 

Network 5: Testing servers
10 virtual machines for testing, mostly for MDM (Apple). In the future we wil use this for manage (MDM) devices for costumers. 

If I’m looking at this PDF: utmshop.nl/.../sophos-sg-series-sizing-guide.pdf and calculate with the devices we have, than the calculation is the next:
Workstations: 18x1 + 7x1,5=28
Phones: 25x1=25
VoIP:  8x1? (Max 8 lines out calling at the same time)=8
VPN: 5x 2 (connected all day)=10
VPN tunnel:  ±10~15x2 (connected all day)=20~30
Costumers: 15~20x1=15~20
Webservers: ±80 a day, so 10 at the same time. 10x1,5=15

So totall is a of 120~150… So if we choose for FullGuard, we need a SG310. 
My question is, do I make the right calculation